While I sometimes see Dutch as an endangered language, reality is that with nearly 30 million speakers worldwide, we're actually doing pretty well. Sadly, this can't be said for the 3000 truly endangered languages of the world - nearly half of the world's total number of languages is on the verge of extinction, and with it, large amounts of human culture are in danger of disappearing forever. In collaboration with several universities and language institutions, Google has launched the Endangered Languages Project to document these languages - textually, visually, and
auditorially.

I dislike the "modern" (RLC) version. The orthography is not particularly Celtic, it is basically Anglo-Cornish with English spelling, and differs greatly from Welsh and Breton. I prefer a certain level of linguistic purity for the sake of diversity (and to undo the effect of English imperialism), so Modern Cornish is a no-go.
The same is true for the Peurunvan and Skolveurieg orthographies (the latter being too french) for Breton. The right one to use is obviously Etrerannyezhel, with its dependency on proper Brythonic etymology. I also prefer formal Welsh over informal Welsh, the former being closer to Cornish and Breton.

But, there's no such thing at Brythonic orthography, nor Celtic. None othe the natural (as opposed to revived) Celtic languages have commonality in their orthography. Gaelic and Irish seriously disagree, and really, they are closely related offshoots of the same language. Welsh is competely different (save the C over K preference and the agreement on CH being a friative). Indeed, Irish and Aelic don't even agree on how to aspirate/leinate (mutate in welsh).... The most Welsh looking Breton orthography is still miles away.

If you look at common phrases (the Pan Celtic phrase book by Y Lolfa,ISBN 0862434416, is a pretty good source) the commonality can be seen, but Welsh and Breton have diverged a lot. Oddly, sometimes Breton is the least Celtic word order, sometimes Welsh. The Gaelic/Gàidhlig/Gaeilge look similar, but with enough subtle grammatical differences so as to not be identical.

Cornish doesn't really exist anymore. All you have is 3 different opinions of what it might look like now. Modern is the closest to reality.

Manx, well, the orthography is exactly what it is.

I think you need to realise, one persons idea rules most written languages. From Ataturk to Kanji. Sometimes it's the orthography that really creates identity. E.g. Thai and Lao, Finnish and Estonian, Gaelic and Irish, Swedish, Danish and Norwegian.